See thro’ this air, this ocean, and this earth
All matter quick, and bursting into birth:
Above, how high progressive life may go!
Around, how wide! how deep extend below!
Vast chain of being! which from God began;
Natures ethereal, human, angel, man,
Beast, bird, fish, insect, who no eye can see,
No glass can reach; from infinite to thee;
From thee to nothing.–On superior powers
Were we to press, inferior might on ours;
Or in the full creation leave a void,
Where, one step broken, the great scale’s destroyed:
From Nature’s chain whatever link you like,
Tenth, or ten thousandth, breaks the chain alike.

Alexander Pope, in this portion from his Essay on Man, demonstrated in lucid prose the social significance that the great chain of being, or scala naturae (“ladder of nature”) had for centuries of philosophers and naturalists. In my earlier post Reexamining Ardipithecus ramidus in Light of Human Origins I showed the flaws in Owen Lovejoy’s reasoning concerning his grand evolutionary narrative of human origins. However, what was nearly as irritating as Lovejoy recycling his unsupported theory from the 80s and using it for a new species, was the repetitive use of the term “missing link” in relation to this fossil discovery.National Geographic highlighted their article on the discovery with the title, “Move over, Lucy. And kiss the missing link goodbye.” On another article they go even further and state, “Oldest ‘Human’ Skeleton Refutes ‘Missing Link.’ Even David Pilbeam, professor of human evolution at Harvard University, told The New York Times that Ardipithecus is important because it begins “to fill in the temporal and structural ‘space’ between the apelike common ancestor and Australopithecus.”

A missing link implies an otherwise smooth gradation in the chain of life, with every space in the cosmos filled and each organism in its proper place. For Pope’s 18th century worldview, and for centuries of thinkers beforehand, it meant that any alteration in the links that bound the system together, whether tenth or ten thousandth, would result in an upheaval of the divine plan. This view of the world was more than simply a way to order nature from highest to lowest; it was also a social philosophy that demanded all economic classes know their place as part of the natural order. It was a vision that permeated all aspects of life and, like Aristotle’s physics or Galen’s medicine, was “known” to be an obvious truth for a millennium.

Arthur O. Lovejoy (no relation to Owen Lovejoy), in his monumental work The Great Chain of Being, demonstrated that this concept rests on the combination of two important ideas that first appear in the works of Plato and Aristotle: “plenitude”, or the idea that nature is complete and coherent in design, and that of “continuity” between one form of life and another. As Lovejoy stated:

From the Platonic principle of plenitude the principle of continuity could be directly deduced. If there is between two given natural species a theoretically possible intermediate type, that type must be realized – and so on ad indefinitum.

Any perceived “gaps” in this design would suggest that the universe was not filled to its fullest extent and was therefore not considered “good” by the assumed creator. Plato argued that it is wrong for us to think “the world was made in the likeness of any Idea that is merely partial; for nothing incomplete is beautiful.” According to this view, every plant and animal must be in perfect balance with this divine order and all aspects of nature must work together as though they each were organs in the body firmament. Aristotle’s “footnote” to Plato was that nature:

passes so gradually from the inanimate to the animate that their continuity renders the boundary between them indistinguishable . . . For plants come immediately after inanimate things . . . And the transition from plants to animals is continuous.

In this way the divine schema consisted of every part of the world filled to its fullest capacity and every gradation of being represented in its proper place. The balance of nature was complete and, from such perfect order, humans could understand their place in the grand design.

This was clearly understood by the Royal Society of London, according to their first historian in 1667, when they combined this Platonic view with the Baconian formulation of empirical data collection to understand and control the great chain of being:

Such is the dependence amongst all the orders of creatures; the animate, the sensitive, the rational, the natural, the artificial; that the apprehension of one of them, is a good step towards the understanding of the rest. And this is the highest pitch of humane reason: to follow all the links of this chain, till all their secrets are open to our minds; and their works advanc’d or imitated by our hands. This is truly to command the world; to rank all the varieties and degrees of things so orderly upon one another; that standing on the top of them, we may perfectly behold all that are below, and make them serviceable to the quiet and peace and plenty of Man’s life.

Plate from Darwin’s On the Origin of Species illustrating natural selection as a branching tree.

However, this vision of divinely ordered perfection was dramatically ripped apart, link-by-link, on November 24, 1859. The publication of Darwin’s On the Origin of Species revealed the inherent flaw in the great chain of being: species were not created and placed into a stable hierarchy following Pope’s “Beast, bird, fish, insect,” rather, each species independently evolved from ancestral forms in a step-wise fashion and were no more “lower” or “higher” than any other. Whereas the great chain saw life arranged horizontally from simpler to complex along a two-dimensional axis, natural selection revealed that life was not a chain but a tree, producing species vertically along a three dimensional matrix from the past to the present. Every species alive today, no matter how “primitive” from our perspective, are the current products of more than 3 billion years of evolution. Horses and horny toads, pandas and people, each species is just as “evolved” as any other, each are the new spring leaves adorning the tree of life.

As a result of this Darwinian revolution, referring to Ardipithecus ramidus as a “missing link” is using an antiquated metaphor completely outside of the worldview that gave rise to it. It would be like referring to a medical breakthrough in the treatment of lung disease by using Galen’s view that it caused a “reduction of phlegmatic humours” in the chest. This primate fossil is not a missing link any more than any other fossil find is, whether they be ancestral to humans or ancestral to turkey buzzards. What the term reveals is nothing more than our human chauvinism implying that we were the one and sole purpose of creation.

The more appropriate way to view Ardipithecus is by referring to it as a “primate of modern aspect,” emphasizing the find as one of the earliest known specimens of an animal that looked more like us. I don’t take nearly as much issue with Pilbeam’s interpretation as I do with National Geographic‘s (since it’s reasonable to assume some kind of intermediate form between a fossil like Darwinius and Australopithecus). However, the repeated use of this idea demonstrates how antiquated notions like a “missing link” or progress in evolution continue to appear even by those who should know better. Whether Ardipithecus was a “transitional form,” an ancestral species of humans, chimpanzees and bonobos, or is simply a twig off the main branch 4.4 million years lower on the tree, will remain to be seen. In this way, Pope’s metaphor still has relevance to our worldview today, even if the philosophy he was referring to has long since gone extinct. Evolution may not operate horizontally, by adding links to a linear chain, but science certainly does. The great chain of being may have no relevance to modern biology, but any links that are missed in the scientific process may fall prey to Pope’s warning.

Where, one step broken, the great scale’s destroyed:
From Nature’s chain whatever link you like,
Tenth, or ten thousandth, breaks the chain alike.

Comments

Very nice essay, Eric. The straight-line version of evolution, unfortunately, still remains prominent despite what we have come to know about the pattern of evolutionary history.

I just have one quibble. In the last paragraph you point out that Ardipithecus ramidus might “simply [be] a twig off the main branch.” Doesn’t the phrase “main branch” invoke the Great Chain of Being to some extent? This kind of imagery was often used by paleontologists in the early 20th century, when orthogenesis and internally-directed modes of evolution were being considered, in which there was a “main line” (leading to living organisms) and many “side branches” which were failed evolutionary experiments. It is only in retrospect that we can look back and crown our lineage as the “main” one because we are the only hominins that have survived. Overall I think it would be better to drop the language of “main” and “side branches”, as difficult as it might be to do, as it still calls up evolutionary imagery that writers like you and I are trying so hard to root out!

@Laelaps: You’re absolutely right! It’s such a common way to think about evolution that it even slips through when you’re consciously trying not to. There is no “main branch” in evolution. If jelly fish were writing the history of life they would interpret the main branch as whatever life forms eventually led to them. Thanks for pointing out my own unconscious human chauvinism!

Eric, as usual you make excellent arguments. It is important for students of fossil primates to keep some perspective. Pressure to relate one’s work to humans likely reduces objectivity.

On that subject – Darwinius. Based on parsimony, there is no intermediate between Darwinius and Ardipithecus because the former is a strepsirrhine and the latter is a haplorhine. Similarities between Darwinius and some haplorhines are due to convergence.

I think the reason it’s so persistent is because it does< contain a grain of truth. There really is a lineage leading up to our species (and hopefully through us to whatever’s next), stemming from apes, monkeys, prosimians, shrew-like creatures, stem-mammals, early tetrapods, fish, worms, sponges, microbes, and bacteria. So there are links in that sense (and A. ramidus could quite conceivably be one of them, although it’s hard to tell for certain). The problem is that focusing on this one lineage causes us to forget that most organisms are not part of it, and that every living organism has its own lineage which overlaps ours to some degree.

This essay is akin to a pedantic diatribe on the unscientific inclusion of “white” or “black” in a description of a creature’s “colors.” Can the well-meaning popular press not use “missing link” as shorthand for a truly intermediate form, as distinguished from a “cousin” thereof, without being gratuitously reminded at considerable length that evolution is not directional? Is assuming we all get that and wanting to move on hierarchist?

@JMGPerry (#6): You’re absolutely right, there is no “intermediate” between Darwinius and Ardipithecus. My point was that in the branching tree of evolutionary change you would expect to find a species that was not quite one and not quite the other, but which shared features of both. I believe this is what Pilbeam meant when he talked about filling the “temporal and structural space.” I could be wrong though. Perhaps I’m being charitable.

Having finally, after years of struggle, converted to cladism, I no longer understand the concepts of “missing link” or “transitional species”, and all ancestors are hypothetical. Am I not the better for it?

Whereas the great chain saw life arranged horizontally from simpler to complex along a two-dimensional axis, natural selection revealed that life was not a chain but a tree, producing species vertically along a three dimensional matrix from the past to the present.

Hmmm…I like the idea of a three dimemnsional representation using time as the axis along which evolution moves, not some predetermined invisible guideline presumably put there by some god.

But I think ordering it vertically, unfortunately, still continues the old prejudice of the later forms being “higher” than the earlier forms, because that is the way they would appear along a vertically oriented graph.

What you are going after here (IMHO) is habits of thinking that have been developed by philosphers, religious thinkers, and even some of the earlier scientists of the 18th and 19th centuries.

That is a big task, trying to change the whole way a civilization habitually views itself and its place in existence. But then again, the history of science is in a deep respect the history of a changing worldview brought about by the process of science itself, so you are actually continuing a kind of tradition of forward looking scientist/philosphers.

It is a challenge. A famous example of course is Einstein, who really did not like the propspect that his own work so deeply questioned the order of a universe in which “God does not play dice”. In the 21st century we can look back and see just how ingrained the old “predetermined order” worldview has been, even among those we consider to be the pioneers who helped get us beyond that worldview.

i have been critical about the description of ardipithecus as ‘the a transitiona; “” proto”human”. in that sense i agree, the wish ro define “a” (transitional) missing link is very often overrated. otoh the sequence of bones will allways have transitional features, if not between hominids (wich appears only more doubtfull over time, for example these days there is talk about contemporain erectus/sapiens/-,neanderthaler) then between primates and humans, primates and great apes, or the different great apes among another (and us). after the discovery of ardipithecus i have been in great excitement about when we would actually start finding pan or gorilla, that we don’t find them may not only mean the seperation took place somewhat(or quite some) later (eg. with boisei for gorilla or whatever, but also that what we find is of the hominid lineage as that apparently did have some behavioral characteristics that facilitated fossilisation.,